As Cdk4 is required for vHa-ras-driven mammary tumorigenesis [106], and given that CDK4R24C females have an increased incidence of spontaneous mammary tumors and that the CDK4R24C protein cooperates with RAS to drive melanoma formation, it is surprising that co-expression of CDK4R24C and vHa-ras in mammary epithelial cells delays the onset of tumorigenesis. This evidence concerns the gene CDK4 and breast cancer.